Short story:
gst-plugin-scanner became arch aware to fix bug 615357.
gst-run needs a similar change, or it will ignore the user's choice of arch.

Long story:

On MacOSX, it is possible to build gstreamer to produce fat binaries that will run on both i386 and x86_64.   Users can choose whether to run the 32 bit
version of binaries on a 64 bit mac with the command
  arch -i386 cmd...
This works fine in the following case:
  arch -i386 gst-inspect-0.10
but fails in the case
  arch -i386 gst-inspect

But the 'arch' command is not transitive; child processes are not restricted to be 32 bits.  
And the 'gst-inspect' command is not really gst-inspect, but gst-run, which
then runs gst-inspect-0.10.

Those two facts together combine to cause "arch -i386 gst-inspect" to run the 64 bit version of gst-inspect-0.10.

This is a problem if you need to run a plugin that is only available in 32 bits, e.g. osxvideosrc.

A fix would be for gst-run to prefix the command being run with arch -i386 when appropriate as gst-plugin-scanner does.

A workaround for the user is to invoke the versioned binary directly.